The United States' addiction to petroleum based fuels is an ever present environmental problem and an increasingly heavy financial burden. Biofuels are one option that would help reduce the dependency upon petroleum fuels. Biodiesel is a type of biofuel that may provide an alternative fuel source to help replace petroleum fuels. Biodiesel is fuel made up of fatty acid alkyl esters, fatty acid methyl esters (FAME), or long-chain mono alkyl esters. Biodiesel can be made from a large array of fatty acids. The list of available raw materials utilized to produce biodiesel ranges from used cooking oil to liquefied chicken fat.
Biodiesel production is commonly done by transesterification of animal or vegetable oils/fats. Such oils and fats comprise triglyceride esters containing long chain fatty acid moieties. In biodiesel synthesis, such triglycerides are transesterified with short chain alcohols, typically methanol and sometimes ethanol, though other alcohols have been used. The reaction can be carried out in the presence of an acidic or basic catalyst and in general the basic catalysts are faster, with sodium hydroxide or potassium hydroxide being most commonly used. Typically, sodium or potassium hydroxide with a relatively low water level is mixed with the alcohol, for example methanol, and the mixture is then mixed with the oil. Glycerol is a byproduct of the transesterification reaction. After the reaction is complete, a neutralizer is used to remove the catalyst from the product.
Once the transesterification reaction is complete, the glycerol byproduct must be given time to settle out. The amount of time required for the glycerol to settle out of the reaction mixture is one limiting factor in the production of biodiesel that causes problems when trying to produce the fuel on an industrial scale. It takes approximately eight hours for the glycerol byproduct to sufficiently settle out of the reaction mixture, such that a suitable biodiesel fatty acid methyl ester may be retrieved. Other factors, such as the speed of the transesterification reaction itself, also present problems in deriving a biodiesel production technique that is efficient enough for industrial scalability.
In the production of biodiesel, the cost of the oil or grease is the single largest component of production costs. Yellow grease, which is used vegetable oil from the fast-food industry, is much less expensive then soybean oil but has a very limited supply. Yellow grease is estimated to cost roughly $1.55 per gallon in the year 2012-2013 versus $2.80 per gallon for soybean oil. In 2011, the U.S. biodiesel industry reached a milestone by producing over 1 billion gallons of fuel. This is just a small amount compared to the 33 billion gallons of on-highway diesel consumed by the United States annually. In addition, the current price of a gallon of soybean oil is up to three times the cost of diesel fuel. Even though the potential market for the glycerol byproduct offsets some of these costs, it still makes biodiesel less economically feasible in comparison to conventional diesel fuels.
Despite the greater price, there are many reasons that biodiesel should still be developed. One such reason is that the exhaust emissions from biodiesel are significantly lower than those of regular diesel fuel. Another is that when biodiesel is added in a 1-2% amount to regular diesel fuel it can give the fuel better lubricating properties. Thus, biodiesel production is still being considered in the United States and Europe as an alternative fuel.
In biodiesel production plants, generally, the reaction takes place in either a batch reactor or continuously stirred tank reactors. With a batch reactor, a 6:1 ratio of alcohol to triglycerides is used and the reactor is operated at around 65° C. The reaction will take anywhere from twenty minutes to an hour to be complete. In some processes, the batch is left in the reaction vessel to initially settle the biodiesel and glycerol byproduct, while in other processes the batch is pumped into a settling vessel or a centrifuge.
Another way biodiesel is produced in industry is through a continuous process system. A continuous system consists of continuously stirred tank reactors (CSTR) in series. The reaction is carried out normally in a first CSTR; afterwards, the initial glycerol is decanted. After this glycerol is separated out, the reaction in a second CSTR occurs at a faster rate with a greater percent completion. The disadvantage of the continuous system is that there must be enough mixing to sustain a continuous composition throughout the reactor. This means that the dispersion of glycerol in the biodiesel phase is greater and, therefore, it will require more time to settle out of the layer.
After the biodiesel and glycerol are produced in the reactor, they must be separated through a unit operation that speeds up the natural settling of the phases. Centrifuge systems, decanters, and hydrocyclones are most commonly used for this purpose. Decanters rely on density differences between the two phases and residence time to achieve separation. However, the size of the decanter is a function of the residence time. This means that the only way of decreasing time of separation is to increase the size of the decanter, which in turn increases the cost of the process equipment. These systems are usually only used in small batch processes, where the longer residence time is acceptable. Centrifugal systems are mostly used in continuous biodiesel processes. The centrifuge separates biodiesel and glycerol by creating an artificial gravity field by spinning at a high velocity. The centrifuge is used extensively in industries, but is expensive due to initial costs and maintenance. Hydrocyclones are considered to be an effective, but still experimental, method of ester-alcohol separation. The liquid mixture enters the hydrocyclone at a high pressure, and then passes from a wide to narrower section of the inverted cone where the pressure decreases and velocity increases. This causes an increase in gravitational forces, which causes the denser liquid, glycerol, to be accelerated towards the wall while biodiesel, the lighter liquid, is concentrated in the center.
Even though most current separation operations use the difference in densities to separate the phases, there are new technologies that use ultrasonic energy. These ultrasonic processing apparatuses achieve separation by applying ultrasonic energy at two different frequencies to the reactant fluid. The sonic waves transmit through the walls and into the reactants where it accelerates the transesterification and separation processes within the tank. These devices can produce biodiesel at a continuous rate, up to one gallon per minute with a power of 5000 watts. While they do increase reaction and separation times, they do have the disadvantage of being very expensive. As of yet, there are no single ultrasonic processors that can handle enough biodiesel that would be produced in a large production plant. Instead, multiple ultrasonic processors would have to be used, leading to a high initial cost.
Biodiesel production time can be lessened by a variety of techniques, including the aforementioned methods of decreasing the settling time of the glycerol byproduct; however, the techniques currently used in the art also increase the cost of production. The cost to benefit ratio of producing biodiesel has been a primary reason for the lack of commercially produced biodiesel in this country.
If a low cost method were available to decrease the production time associated with the manufacture of biodiesel, then such a method would have a great impact, making the production of biodiesel on an industrial scale commercially viable.
Thus, there is a need in the art for the development of a biodiesel production method that decreases the time of producing the fuel. In particular, methods that decrease the amount of time for the glycerol byproduct to settle out of the reaction mixture are needed. Furthermore, methods of decreasing the settling time of the glycerol byproduct are needed, which do not rely upon expensive machinery such as centrifuges, decanters, and ultrasonic devices, and the energy they require.